Microsoft doesn't support Windows 7 any more, why should WhatsApp?
Unrelated to market share & OS updates, there's the technical question. There aren't many Win32 APIs which are present in Windows 8, but not Windows 7. Thus it's not that likely that Windows 7 wouldn't be supported by a Win32 app (which this is), unless they go out of their way to make it so.
[1] http://windows.microsoft.com/en-us/windows/lifecycle
[2] http://www.w3schools.com/browsers/browsers_os.asp
[3] https://www.netmarketshare.com/operating-system-market-share...
According to Wikipedia I'm not: https://en.wikipedia.org/wiki/Windows_7#Support_lifecycle
"Mainstream support for 7 ended on January 13, 2015."
I think it's reasonable that businesses don't support an OS version beyond the OS's provider's own "mainstream support".
> Thus it's not that likely that Windows 7 wouldn't be supported by a Win32 app (which this is), unless they go out of their way to make it so.
Possibly, but listing support on their website means they support it, and why should they do that if, again, Microsoft itself does not?
Extended support will end on January 14, 2020